In a common rail-type fuel supply system of a diesel engine in the related art, fuel from a fuel tank passes through a fuel filter to be supplied to a common rail through a high-pressure pump. In the common rail, fuel that is not injected to an injector returns to the fuel tank via a fuel return line. On the other hand, fuel that is discharged through an outlet of the pump returns to the fuel tank via the fuel return line after lubricating the high-pressure pump.
FIG. 1 shows a realization of the above-mentioned fuel supply system in an actual engine. As shown in FIG. 1, a fuel filter can be improperly assembled. If an operator incorrectly assembles the fuel filter, a fuel supply line and a fuel return line are not to be connected correctly, thereby making the fuel to be retrieved through the fuel return line lose its fluidity and supplying unpurified fuel to the high-pressure pump.
If the engine is operated under the condition mentioned above, pressure in the fuel return line is to increase gradually. When the pressure in the fuel return line is about 2 bar or more, parts forming the fuel supply system tend to deform to cause damages to the fuel supply system and fuel leakage.